The present invention relates to a method for producing a plastic film having improved characteristics and to the film thus obtained.
The invention also relates to apparatuses suitable to perform the method.
Plastic films are currently widely used, particularly for packagings which are used mainly to package food products.
Plastic films are in fact capable of combining easy workability and versatility with appreciated characteristics of impermeableness, great strength and low cost.
Recently, the trend in the market of food products is increasingly toward plastic films having improved characteristics in order to protect the contained product in the best possible manner and for a longer time and in order to allow printing on said film.
This is the case, for example, of films to be used to produce packages on which pictures, figures, decorative and ornamental patterns must be reproduced and/or it is necessary to highlight the manufacturer's trademark; such packages require particularly easy adhesion of inks and printing dyes as well as of the adhesives used in packaging processes.
Some packages instead particularly require the characteristic known as “barrier effect”, owing to which the product, as long as it remains inside the package, is isolated from the outside by means of materials which reduce the exchange of moisture and gases with the environment and shield against externally-originating UV rays.
In recent times, so-called “smart” packagings have also been devised: they owe their name to the fact that they can be activated after their production (for example by irradiation with UV rays, infrared rays, or electromagnetic fields); by way of said activation, they can acquire new properties, such as for example the ability to react with oxygen.
Packagings of this type are applied in particular for preserving foods, since by absorbing the oxygen that is present inside the package they prevent the proliferation of germs and bacteria and reduce oxidative reactions.
Currently, the production of plastic films having these improved characteristics can be based on the surface application of substances on plastic films at the time of their use, i.e., long after their production.
This method of treating the film entails considerable technical difficulties if particular characteristics are desired.
Once the extrusion process has ended, the plastic film is in fact substantially unsuitable for the surface adhesion of the layer of “active substance” due to lack of wettability or anchoring.
It is therefore necessary to resort to special bonding agents and/or additional treatments which in any case are insufficient to ensure full and firm adhesion of the substance to the film but help to increase the cost of the process for producing this type of film having modified characteristics.
It is also possible to heat the film, so that its surfaces become more suited to the treatment, but the temperature must not be high or close to the softening value in order to avoid problems in terms of irreversible deformations.
Additionally, a film kept in store continues to vary its structural characteristics over time, so that after some time, even if the film is returned to the same temperatures, no identical behaviors are obtained.